Process for producing membrane for capacitive vacuum measuring cell

ABSTRACT

A ceramic membrane for a capacitive vacuum measuring cell includes a thin ceramic membrane with a thickness of &lt;250 μm, in particular less than 120 μm. The membrane is produced from a ribbon-shaped green body of Al 2 O 3 , and is given high planarity by smoothing the membrane after sintering. The green body is sintered at a sintering temperature that is higher than the smoothing temperature applied following sintering.

BACKGROUND OF THE INVENTION

The present invention generally relates to a membrane for a capacitivevacuum measuring cell.

It is known that pressures or pressure differences can be measured byapplying pressure to a thin membrane and measuring its deflection. Aknown and suitable method for measuring the deflection is to design themembrane arrangement as a variable electrical capacitance, where thecapacitance change which correlates with the pressure change isevaluated by measurement electronics in the known manner. Thecapacitance is created by arranging a thin, flexible membrane very closeto another surface and by depositing an electrically conductive film onboth mutually opposed surfaces or by fabricating them from electricallyconductive material. When pressure is applied to the membrane, thedeflection changes the distance between the two electrodes which leadsto an analyzable capacitance change of the arrangement. Sensors of thistype are mass-produced from silicon. The flat basic body, as well as themembrane, often consist entirely of silicon. There are also versionsthat are made of composite materials such as silicon with a glasssubstrate. Such sensors can be produced very economically. However, invacuum applications, pressure sensors of this type are normally usableonly for higher pressures in the range of approx. 10⁻¹ mbar to severalbar. High resolution at pressures below 10⁻¹ mbar is no longerachievable with silicon. One of the reasons for this is that the siliconsurface reacts with the environment, which impairs the sensitive sensorcharacteristic. Already water vapor that forms part of normalatmospheric air leads to a corresponding reaction on the surfaces. Theproblem becomes even more serious when the sensor is used in chemicallyaggressive atmospheres. For this reason, attempts were made to protectsuch silicon sensors against external influences by passivating thesurfaces. Attempts were also made to deposit protective coatings on thesurfaces in order to improve the durability and the resistance againstchemically aggressive environments as described in DE 41 36 987. Suchmeasures are costly and, in the case of mechanically deformable partssuch as membranes, have only limited success, in particular in highlyaggressive media such as fluorine, bromic acid and their compounds whichare typically used in vacuum etching processes.

For this reason, attempts were made to build vacuum measuring cellsentirely from corrosion resistant materials such as Al₂O₃. A knownarrangement of this type is shown in FIG. 1. The vacuum measuring cellconsists of a ceramic plate (20) above which a membrane (22) is arrangedwith a small gap between the two of them and a fusible seal (21) betweenthe ceramic plate (20) and the edge of the membrane. The ceramic plate(20) together with the membrane (22) forms a reference vacuum chamber(25) that is evacuated down during the manufacturing process through apumping port and which is sealed with a seal (28). The mutually opposedsurfaces of the ceramic plate (20) and the membrane (22) inside thereference vacuum chamber (25) are coated with electrically conductivematerial and connected to insulated external terminals on which thecapacitance signal can be evaluated by means of an electronic device(not shown in the illustration). To achieve corrosion resistance, plate(20) and membrane (22) are both made of ceramic material such as Al₂O₃.This vacuum measuring cell in turn is arranged in a vacuum-tight housing(23) that features a port (24) which is connected to the media to bemeasured. Via port (24) of the vacuum measuring cell, the resultingmeasurement vacuum chamber (26) is sealed off against the membrane (22)by means of an elastomer seal (27) so that the pressures to be measuredreach only the surface of the membrane (22). For the purpose of sealing,the entire cell is pressed via the ceramic plate (20) and membrane (22)against the elastomer seal (27). Up to now, vacuum measuring cells ofthis type have been usable only for higher pressures in the range of 0.1mbar to 100 bar. In addition, this design leads to stress in thematerials which, at lower pressures, for example <1 mbar, significantlyimpairs the reproducibility of measurement results and the resolution.The ceramic membranes (22) used so far have a thickness ranging from 279μm to 2540 μm. Such designs are not suitable for achieving widemeasurement ranges, in particular low pressures of 0.1 mbar to 10⁻⁶mbar. In addition, designs of this type, as disclosed also in U.S. Pat.No. 5,553,502, are costly.

The objective of the present invention is to eliminate the disadvantageof the current state of the art. In particular, the objective of thepresent invention is to implement an easy-to-produce, economicalmembrane consisting of Al₂O₃ for a vacuum measuring cell that issuitable for pressures from 10⁻⁶ mbar to 1000 mbar, in particular from10⁻⁶ mbar to 1 mbar, with an accuracy of better than 1%, preferablybetter than 0.3% of the measured value. The measurement range can becovered or subdivided into several vacuum measuring cells or membraneversions according to the invention. In addition, this vacuum measuringcell shall be corrosion resistant to aggressive media, have a compactdesign, and be economical to manufacture.

SUMMARY OF THE INVENTION

With respect to the generic membrane, the problem is solved through thecharacteristic features of patent claim 1. The dependent patent claimsrelate to preferable other versions of the invention.

The capacitive membrane according to the invention is made entirely outof ceramic, in particular Al₂O₃. In consequence, it is now possible tobuild a vacuum measuring cell entirely out of corrosion resistantmaterial, preferably out of Al₂O₃. This results in high corrosionresistance and long term reproducibility. Only in the areas wheresealing is required, or where feedthroughs are provided, are smallamounts of materials other than Al₂O₃ used, if the Al₂O₃ is not fusedwithout addition of the foreign material. A vacuum measuring cellconsists of a first plate-shaped housing body above which a membrane,sealed along its edges, is arranged so that it encloses a referencevacuum chamber. On the side pointing away from the reference vacuumchamber, there is a second housing body, also sealed along its edges, sothat a measurement vacuum chamber is formed there. This measurementvacuum chamber features a port for connecting the medium to be measured.The surface of the first housing body and the membrane that form thereference vacuum chamber are coated with an electrically conductivefilm, for example, gold, and constitute the electrodes of thecapacitance measuring cell. The electrodes are led out, for example,through the first housing body or through the sealing area in the edgezones. The essentially parallel electrode surfaces are spaced apart from2 μm to 50 μm. Sealing of the membrane in the edge zone against the twohousings is preferably achieved through welding, for example, laserwelding. Highly suitable, and simple to use, is also a glass brazingmaterial that is corrosion resistant. Another possibility of achieving asealing bond is to connect the housing parts diffusively, for example,in the green body state in which the objective is to completely avoidmaterial other than Al₂O₃.

The membrane according to the invention in the measuring cell mentionedabove essentially allows a symmetric design that avoids all stress inthe housing. This is particularly important in order to achieve highmeasurement sensitivity combined with high measurement accuracy andreproducibility at low pressures. It also allows the utilization of avery thin ceramic membrane according to the invention, which isessential for reliably measuring vacuum pressures lower than 100 mbar,and in particular lower than 10 mbar, by means of capacitive,all-ceramic measuring cells. For this purpose, membrane thicknesses of10 μm to 250 μm are needed, where membrane thicknesses of 10 μm to 120μm are preferred in order to achieve a very good resolution. Typicalmembrane thicknesses are, for example:

at 1000 Torr: membrane thickness 760 μm±10 μm

at 100 Torr: membrane thickness 345 μm±10 μm

at 10 Torr: membrane thickness 150 μm±10 μm

at 1 Torr: membrane thickness 100 μm±10 μm

at 0.1 Torr: membrane thickness 60 μm±10 μm

at 0.01 Torr: membrane thickness 40 μm 10 μm

Such thin membranes are very difficult to manufacture and, after thesinter step, require at least one additional smoothing step. It is alsovery important for the membrane to be sufficiently helium tight whichcan be achieved only if the grain size of the membrane material is nottoo large and remains within the range of <20 μm. Smaller grain sizes of<10 μm are preferred, in particular, those <5 μm. In all cases, thecross-section of the membrane viewed across the thickness should containat least two grains; the membranes are particularly tight if more thanfive grains are on top of each other.

Another important criterion for achieving an accurate measuring cell isthe planarity of the membrane surface. The unevenness across the entiresurface should in all cases not exceed 30% of the electrode gap,preferably no more than 15%. This means that the unevenness across theentire surface should not exceed 10 μm, preferably not more than 5 μm.The unevenness is defined here as the difference between the lowest andthe highest point. To achieve the desired long-time stability, thepurity of the aluminum oxide used for the membrane should be at least94%, with preferred values being above 99%.

To ensure that the quality of the membrane seal in the edge zone is notimpaired, it is advantageous to lead out the electrically conductivelayers via feedthroughs that are arranged on the first housing body,rather than directly through the membrane seal or fused joint.

To ensure accurate functioning of the measuring cell over a long periodof time, the reference vacuum chamber must have a high-quality vacuumwith long-time stability. After evacuation, a getter should be providedthat is preferably arranged in a small volume on the first housing andcommunicates with the reference vacuum chamber. This getter ensures thatthe reference vacuum pressure is lower, preferably by at least onedecade, than the pressure to be measured. To prevent contaminations ofthe internal measuring cell space, a getter type that is not evaporatingshould be chosen.

Measuring cells designed according to the invention can be very compactand economical to produce. The diameter of such a cell can be between 5and 80 mm where the measuring cell preferably has a diameter of 5 to 40mm. The thickness of such a cell is preferably in the range of 2 mm to25 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art drawing of a known vacuum measuring cell builtmainly from corrosion resistant materials such as Al₂O₃.

FIG. 2 shows a schematic cross-section of a capacitive vacuum measuringcell according to the invention.

FIG. 3 shows an enlarged cross-sectional detail according to FIG. 2.

FIG. 4 shows a temperature/time diagram for the sintering step of themembrane.

FIG. 5 shows a temperature/time diagram for a smoothing step of themembrane.

FIG. 6 shows a membrane resting between two flat plates, and compressedtherebetween.

FIG. 7 shows a membrane cast from a thin strip of Al₂O₃ green bodysupported on a carrier foil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For manufacturing a functional measuring cell that possesses theaforementioned characteristics, the specifications of the correspondingmanufacturing process must be closely followed. In particular, themanufacture of thin ceramic membranes requires special care. Themembrane as well as the complete unit should be entirely free ofinternal stress.

Suitable Al₂O₃ membranes according to the invention are manufactured byfirst mixing a slurry according to a specific recipe, and by thinly andevenly spreading the doughy mass on a strip shaped carrier material, forexample, a plastic foil. After drying, these layers are inspected forirregularities such as bubbles or pits. This mass, which is not sinteredyet, is referred to as the green body. The desired membrane shape is cutout of the strip shaped green body 106 (see FIG. 7) material, afterwhich the material is still sticking to the plastic foil 108 (see FIG.7). For cutting, tools such as knives or punching tools are used, or alaser. Cutting or scoring of the green body requires particular carethat no dislocations or warping against the surfaces of the futureceramic membrane occur, as this also influences the degree of surfaceunevenness. If a cutting knife is used, a pressing wheel can be appliedon the membrane side which prevents undue warping of the green body.Subsequently the preferably circular cut membranes are separated fromthe foil strip by drawing off the latter, for example, across an edge.The membranes are subsequently sintered in a furnace. For sintering, themembranes are preferably placed on hard-sintered, flat Al₂O₃ plates thatcan be on top of each other, and sintered typically at 1630° C. Thetemperature is gradually raised to 1630° C. over a period of approx. 400minutes, which corresponds to a temperature rise of about 4° C. perminute. The temperature is then held for a few minutes at this level,for example, 6 minutes, and then slowly decreased at the rate of about3° C. per minute over 210 minutes to 1000° C., and in a second step,with a temperature reduction of about 6° C. per minute over about 170minutes, to room temperature. The result is a ceramic membrane which, incontrast to the green body, has a hard pure ceramic structure, and theadditives of the green body material have evaporated. After thissintering step, the membrane is very uneven and, at a diameter of 40 mm,has a warpage of several millimeters.

In this condition, the membrane cannot be used due to the strong warpageand internal stress in the material. The membrane must be smoothed in atleast one additional step. For this purpose, the membrane is againheated in the furnace. The membrane is carefully sandwiched betweenmassive and highly planar, hard-sintered Al₂O₃ plates (also “dead”, thatis, large-grained Al₂O₃) which, for a membrane diameter of 40 mm, have aweight of several 10 to several 100 grams, or in the example about 60grams, or are correspondingly weighted down. The temperature is slowlyincreased at 4° C. per minute over 390 minutes to approximately 1570° C.After a short dwell time of several minutes, approximately 25 minutes atthis temperature, the temperature is lowered slowly at approx. 5° C. perminute over approx. 115 minutes until 1000° C. are reached.Subsequently, the temperature is lowered at approx. 6° C. per minuteover about 166 minutes until ambient temperature is attained. After thissmoothing step, the membrane has only a very small amount of residualwarpage of a few tenths of a millimeter. Important in this smoothingstep is that the temperature is not raised as high as in the firstsintering process, preferably up to a temperature which is at most 100°C. lower than the sintering temperature. To achieve excellent resultsrequired for the measuring cell to be built, this smoothing heating stepmust be performed at least twice. For reasons of economy, thesesmoothing steps should be performed in such a way that no more than twosuch steps are needed. Particularly good results are achieved when themembrane is carefully separated from the plate between heating steps andredeposited in a slightly offset position. Preferably, the membrane iseven placed upside down. The utilization of a stack of several flatplates with membranes sandwiched in between is particularly economical.

The functionality of measuring cells designed as described above isdecisively influenced by the membrane. Said manufacturing process allowsthe production of thin membranes with high density and good planarity.Strict adherence to the corresponding parameters during the sinteringand subsequent smoothing steps is essential. During sintering, maximumtemperatures from 1300 to 1800° C., preferably 1400 to 1700° C., must beattained. This maximum temperature should be achievable at leastbriefly, but maintained in this range no longer than 180 minutes. Theheat-up rate should not exceed 25° C. per minute. Preferably, theheating phase is subdivided into two steps: After a temperature of 1000to 1300° C. has been attained, further heating to the final temperatureshould take place at a maximum of 15° C. per minute. When said maximumtemperature has been attained and the dwell time maintained, themembrane is cooled off again at a rate of no more than 25° C. perminute. If the heating and/or cooling is performed too quickly, themembranes become wavy and porous. Longer times are not harmful, but areuneconomical.

During the smoothing step, the same conditions have to be maintained asin the sintering step, and the smoothing temperature is not to exceedthe sintering temperature at any time. Preferentially, the maximumsmoothing temperature should stay at most 100° C. below the maximumsintering temperature.

Membranes are now available that have selectable thicknesses in therange of 10 μm to 250 μm, preferably <120 μm. With the process describedabove, membrane planarities can be achieved that are better than 10 μmacross the entire surface, preferably even better than 5 μm. The meangrain size in the membrane material is less than 20 μm, preferably lessthan 10 μm, and even less than 5 μm is achievable. This means that therequirement that at least 2 grains, preferably at least five grains,exist across the thickness, can easily be achieved. In this way, heliumtight membranes, as required for vacuum measuring cell applications, canbe produced. The membrane is now ready to be used for building themeasuring cell.

The membrane, as well as a flat surface of the first housing body madeof Al₂O₃, are now coated with an electrically conductive film forcreating the electrodes. For example, a metallic paint, for example apaint containing gold, can be used which, for example, is brushed orsprayed, preferably printed on. Another method is to create theelectrically conductive layer by means of evaporation coating,preferably by sputter coating. To allow the deposition of an accurateand defined film, it is advantageous if, for example, a gold layer thatinitially is deposited with a relatively large thickness of about 1 μm,is subsequently thinned down in the inner area to approx. 5 nm by meansof an etching process, preferably an ion etching or sputter etchingprocess. In this way, a thicker edge area is created which cancompensate diffusion losses if, for example, a brazing step isperformed. A preferred process that is simple to handle in practice isto first deposit a thin layer of several nm across the entire surfaceand subsequently a thicker layer of gold at the edge by means of screenprinting (that is, a combination process and different filmthicknesses). Membranes or housings processed in such a way aresubsequently tempered at temperatures of several 100° C., preferably inthe range of 650° C.

The second ceramic housing which is arranged on the measurement sideconsists of a flat ceramic plate which, on the membrane side, can have aflat recess in order to form a sufficiently large vacuum chamber. Theconnection port is connected to this ceramic housing by means ofbrazing, bonding or gluing, preferably by means of glass brazing, insuch a way that the connection opening can communicate with the futuremeasurement vacuum chamber.

In the peripheral area where the seal is created, the membrane is coatedon both sides with a glass paste, preferably by means of screenprinting. After drying, the membrane with the glass paste is baked in anoven at several 100° C., preferably at about 670° C. Subsequently, theglass surface is polished on both sides and, thereby preferably also,the future electrode spacing is defined.

With the aid of said coating process, the upper ceramic housing on theelectrode side can, on the external surface, additionally be coated withan electrically conductive film in order to achieve shielding. Alsohere, the connection points are formed on the housing. In an additionalstep, the drill holes for the electrical feedthrough of the electrodeconnections are metallized, preferably with silver.

In a test phase, the first housing with the electrode and thefeedthroughs, together with the deposited membrane, is checked fortightness and for correct electrode distance. Subsequently, the lowerhousing part is mounted, and the entire assembly is loaded with weightin order to test the function and distances. In a mounting frame, thegetter connection may additionally be mounted, and under a load weightof about 200 grams, the glass seals are baked at several 100° C.,preferably about 630° C. Subsequently, a test is performed to check thatthe required distances are maintained. If necessary, the membranespacing can be corrected through additional weight loading or relieving,and an additional firing process. The sealing process must be executedvery carefully and, as mentioned, no stress should occur in themeasuring cell arrangement. Alternatively also, direct bonding can beused in place of glass or other sealants, preferably laser bonding.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure, and are entirely based on the Swiss priorityapplication no. 1997 2954/97 filed Dec. 23, 1997.

The invention is described schematically based on the followingillustrations which serve as examples:

FIG. 2 shows a schematic cross-section of a capacitive vacuum measuringcell according to the invention.

FIG. 3 shows an enlarged cross-sectional detail according to FIG. 2.

FIG. 4 shows a temperature/time diagram for the sintering step of themembrane.

FIG. 5 shows a temperature/time diagram for a smoothing step of themembrane.

A capacitive measuring cell made of a membrane of Al₂O₃ according to theinvention with a structure essentially symmetrical about the membrane isillustrated by the cross-section in FIG. 2. The first housing (1)consists of a ceramic plate made of Al₂O₃ which along its edges istightly bonded at a distance of 2 μm to 50 μm relative to the ceramicmembrane (2) and which encloses a reference vacuum (25). The distancebetween the two surfaces is usually established directly during theassembly by means of the sealing material (3) located between themembrane edge and the housing. In this way, a completely plane housingplate (1) can be used. In the same way, a measurement vacuum chamber(26) is formed in a second housing (4) on the opposite membrane side;this vacuum chamber is accessible for the media to be measured via aconnecting port (5) through an opening in the housing (4).

FIG. 3 shows an enlarged cross-sectional detail of the edge zone of ameasuring cell. The seal (3) on both sides of the membrane (2) defines,as mentioned above, the distance of the two housings (1 and 4). Thisseal consists, for example and preferably, of glass paste that is easyto handle and can, for example, be applied by means of screen printing.In a typical measuring cell with an external diameter of 38 mm and afree internal membrane diameter of 30 mm, the distance (3) is approx. 2to 50 μm, preferably 12 to 35 μm. In this example, the first housing (1)has a thickness of 5 mm, and the second housing (4) a thickness of 3 mm.The inner area of the second housing (4) is preferably designed with anapprox. 0.5 mm deep recess, as shown in FIG. 2, in order to enlarge themeasurement vacuum chamber (26). On the reference vacuum side, themembrane (2) and the housing (1) are each coated with an electricallyconductive film (7). These two films are not electricallyinterconnected. Films (7) can, for example, be painted on, printed on,sprayed on, or be deposited by means of a vacuum process. Preferably,they are deposited by a vacuum process such as by evaporation coating orsputtering. Particularly suited as a film material is gold, which isdeposited, for example, with a film thickness of 1 μm and issubsequently thinned down to a few nanometers, preferably to 5 m, bymeans of sputter etching. In this way, the film thickness can be definedso that it is thin enough and is free of stress. The electricalconnections of the membranes (7) are preferably established withvacuum-tight, electrically conducting feedthroughs (6), preferablythrough the housing (1) where they can subsequently be connected to theevaluation electronics. The evacuation line which leads through thefirst housing plate (1) and the getter arrangement are not shown in FIG.3.

An example of an optimized time/temperature profile for the membranesintering step, beginning with the green body, is shown in FIG. 4.Important for the sintering process is that the membrane material aftersintering has a mean grain size not greater than 20 μm, preferably notgreater than 10 μm, preferably smaller than 5 μm in order to achievehigh gas or helium tightness of the thin membrane. In the cross-section,across the thickness of the membrane, at least 2 grains, preferably 5grains, should be present. After the sintering step, the membrane isbuckled by several mm relative to a plane surface. In this condition,the membrane is not usable and must be smoothed; that is, planaritiesmust be achieved that deviate by no more than 10 μm from the ideal planesurface, preferably less than 5 μm. This is achieved by carefullyheating the membrane which is deposited on a plane surface, in a furnaceup to the softening point. Already due to its own weight, it will adaptitself to the plane base. To achieve better results, this process can berepeated, and the membrane 2 (see FIG. 6) can additionally be smoothedby sandwiching it between flat plates 100 and 102 (see FIG. 6). Aparticularly simple solution is to press the membranes with the weightof the plates themselves, which is in the range of several 10 grams.Between the heating steps performed for the purpose of smoothing, themembranes should be detached from the flat plates and redeposited in adifferent direction or upside down. As a rule, two smoothing steps aresufficient and should be attempted for reasons of economy.

An example of an optimized time/temperature profile for a smoothingstep, starting with a sintered membrane, is shown in FIG. 5. In thisstep, the membranes reach a maximum temperature of 1570° C. and remainbelow the sinter temperature of 1630° C. The membrane becomes soft andcan adapt itself to the plane base, either through its own weight or bymeans of weight loading.

I claim:
 1. A process for producing a membrane for a capacitive vacuum measuring cell that comprises the following steps: a. forming a membrane from a substantially pure Al₂O₃ slurry; b. heating the membrane a first time in a furnace to sinter the membrane; c. cooling the membrane following step b; d. heating the membrane a second time to smooth the membrane; and e. cooling the membrane following step d.
 2. A process according to claim 1 including the step of heating the membrane a third time following step e. for smoothing the membrane.
 3. A process according to claim 1 wherein the membrane has a sintering temperature, and wherein the step of heating the membrane a second time to smooth the membrane is performed at a temperature that is sufficiently high to soften the membrane but which does not exceed the sintering temperature.
 4. A process according to claim 1 wherein the step of heating the membrane a first time is performed at a first temperature; wherein the step of heating the membrane a second time is performed at a second temperature; and wherein the first temperature is greater than the second temperature.
 5. A process according to claim 1 in which the membrane rests on a flat plate during the second heating step, preferably between two flat plates, and is smoothed, preferably by pressing such as by weight loading.
 6. A process according to claim 5 in which the membrane is detached from the plates after the second heating step, redeposited in an offset position upon said flat plate, and subsequently heated a third time following step e. for smoothing the membrane.
 7. A process according to claim 5 in which the flat plate is made of a material that has essentially the same temperature coefficient as the membrane material, and in particular, consists of Al₂O₃, wherein the flat plate material is preferably hard sintered and has a higher softening temperature than the membrane.
 8. A process according to claim 1 in which the membrane shape is cast or pressed from the Al₂O₃ slurry, preferably from a thin strip of an Al₂O₃ green body supported on a carrier foil, and which is subsequently pulled off the foil.
 9. The process recited by claim 1 including the further step of bonding the membrane in a capacitive vacuum measuring cell to enclose a reference vacuum.
 10. A process for producing a plurality of membranes for capacitive vacuum measuring cells that comprises the following steps: a. forming membranes from a substantially pure Al₂O₃ slurry; b. heating the membranes a first time in a furnace to sinter the membranes; c. cooling the membranes following step b; d. heating the membranes a second time to smooth the membranes; and e. cooling the membranes following step d; wherein the step of heating the membranes a second time includes the step of stacking a plurality of membranes between flat plates for the purpose of thermal treatment to achieve smoothing.
 11. The process recited by claim 10 including the further step of bonding each of the membranes in a capacitive vacuum measuring cell to enclose a reference vacuum. 